i NOTE

Monomeric OH Stretching Frequencies in Monohalogenated Monocarboxylic Acids*

J. E. Katon and Deepali Sinha

Chemistry Deportment, Miami University, Oxford, Ohio 45056

(Received 15 January 1971; revision received 22 February 1971)

INDEX HEADINGS : Monohalogenated monocarboxylic acid ; Monomer OH vibration frequencies; Acids, infrared spectra of.

In a recent publication f rom these laboratories we discussed spectra-s t ructure correlations for pure mono- halogenated monocarboxylic acids and noted tha t the bonded OH stretching vibrat ion appeared to undergo a f requency shift in certain of the compounds s tudied? The extreme broadness of these absorpt ion bands prevents accurate peak frequency measurements . We have now measured the frequencies of the monomeric OH stretching frequencies of these compounds in carbon tetrachloride solution, along with those of a few unsubst i tu ted acids for comparison, and report the results here.

The spectra were recorded on 0.1M solutions of the acids in CC14 using a sodium chloride cell with a pa th length of 0.5 m m and a Pe rk in -E lmer model 180 ir spect rophotometer which had been previously cali- bra ted with indene} The scan rate was adjusted to give a 1-h t ime for the entire 4000-200-cm -~ region and an abscissa expansion of 2.5 was used. At this scan rate, frequencies can be read from the frequency readout to =t=0.2 cm-k Variable ordinate expansion (one to five times) was used, the expansion being adjusted so tha t the band height was approximate ly 30% in t ransmi t tance units (about 80% to about 50% from background to peak). The results are given in Table I. The es t imated frequency error is :=t=0.5 cm -~ at a max imum and is p robably less than this.

One correlation is apparen t from the da ta of Table I which could be useful for diagnostic studies. All acids which have a halogen £ tom subst i tuted in either the alpha or beta position have a ~0H monomer fre- quency below 3530 cm -~, irrespective of the halogen a tom (the entire range is 3524-3529 cm-1). All acids

* This work has been partially supported by the U. S. Air Force under contract No. F33615-70-C-1021.

Table I. Tabulation of veil (monomer) and pK~ for several mono- carboxylic acids.

~OH (monomer)

Acid (cm -1) pKA ~

Acetic 3537 4.75 Propanoic 3537 4.87 Butanoic 3535 4.81 Pentanoic 3535 4.86 Hexanoic 3535 4.88 2, 2-Dimethyl propanoic 3536 5.05 Chloroacetic 3524 2.86 Bromoacetic 3524 2.90 Iodoacetic 3527 3.17 2-Chloropropanoic 3525 2.83 2-Chlorobutanoic 3524 2.85 2-Broraopropanoic 3527 2.96 2-Bromo-3-methylbutanoic 3524 2-Bromopentanoic 3525 2-Bromohexanoic 3525 2-Chloro-4-methyl pentanoic 3525 3-Chloropropanoic 3528 4.07 3-Bromopropanoic 3528 4.02 3-Chloro-2, 2-dimethyl propanoic 3528 3-Iodopropanoic 3529 4.04 3-Chlorobutanoic 3527 4.04 4-Chlorobutanoic 3532 4.52 5-Chloropentanoic 3533 4.69 6-Bromohexanoic 3532 11-Bromoundecanoic 3534

Handbool¢ of Organic Structural Analysis, edited by Y. Yukawa (W. A. Benjamin, New York, 1965).

with the halogen subst i tu ted at a carbon fur ther removed from the carboxyl group have a , O H mono- mer frequency above 3530 cm -1 (the entire range is 3532-3534) and which is close to those for unsubsti- tu ted acids (3535-3537).

I t has been claimed tha t a linear relationship exists between the pKA of subst i tu ted acetic acids and their monomeric OH stretching in CC14. 3 This has been discussed by Bel lamy 4 and da ta supposedly confirming the earlier results have been published. 5 Both these papers contained very few data on subst i tuted or monohalogen subst i tuted acids. As a result, the linear relationships were drawn from acids of relatively widely differing pKA'S. Most unsubst i tu ted and mono- halogen subst i tuted acids have pKA's varying be- tween about 2.8 and 5.0, however. Selected pKA values, t aken f rom a s tandard reference work, are also listed for the acids studied in Table I. There is clearly no simple correlation between ~OH (monomer) and pKA. An example is furnished by comparison of 2- and 3-chloropropanoic acids. The , O H of the two

APPLIED SPECTROSCOPY 49'7

compounds differs by only 3 cm -1 while the pKA differs by 1.24 units. A further example is furnished by iodoaeetic acid and 3-chloropropanoic acid. The ~OH value is the same for the two, but their pKA's differ by 0.90 units. Although the stronger acids in this series do tend to have lower ~OH values, the relationship is clearly not a simple linear one. Finally, it has been pointed out 6 tha t substitution of more than one methyl group on the alpha carbon of acids with the structure XCH2CO2H (where X does not necessarily equal alkyl) increases the value of pKA appreciably. I t is clear from Table I tha t the ~,OH frequencies of propanoic and 2, 2-dimethyl propanoic acids, and those of 3-choropropanoic and 3-chloro- 2, 2-dimethyl propanoic acids are essentially the same.

Although Goulden 3 feels tha t there should be a relationship between pKA and ~OH (monomer) and Bellamy et al. a t tempt to rationalize such a relation- ship, we feel tha t there is no valid reason to expect any such correlation as tha t proposed by these authors. The pKA of an acid is determined by the

thermodynamic properties of at least three different species in an aqueous solution. The vOH (monomer) frequency is primarily determined by the OH stretch- ing force constant (with small corrections for vibra- tional coupling and solvent effects). Furthermore, the molecule being studied in CC14 solution is different from that present in pKA measurements. The former is a monomer surrounded by CC14 molecules, while the latter is a hydrated monomer since the measure- ments are made on dilute solutions. Any relationship between ,OH and pKA would therefore be expected to be quite subtle and complex.

1. J. E. Katon, T. P. Carll, and F. F. Bentley, Appl. Spectrosc. 25, 229 (1971).

2. R. N. Jones and A. Nadeau, Spectrochim. Acta 20, 1175 (1964).

3. J. D. S. Goulden, Spectroehim. Aeta 6, 129 (1954). 4. L. J. Bellamy, Advances in Infrared Group Frequencies

(Methuen, London, 1968). 5. L. J. Bellamy, A. R. Osborn, and R. J. Pace, J. Chem. Soc.

1963, 3749 (1963). 6. J. F. J. Dippy, Chem. Rev. 25, 151 (1939).

Position Wanted

Spectrochemist : B. S. 1958, experience in process and quality control, R&D, and supervision. Publications. Emphasis on emission, x-ray, and separation, technique. Background ir-uv, GC-MS, DTA-TGA, and wet analysis. Available immediately, any location. Address queries to Editor of Applied Spectroscopy, 9790 South Cass Avenue, Argonne, Illinois 60439.

Applied Spectrochemist: 25 years experience in applied re- search, development, and analytical work using molecular, atomic-absorption, atomic-emission, and ir spectrographic tech- niques. Experience with stable isotopes using unsophisticated instrumentation and relatively simple techniques. Position wanted with reliable establishment, will relocate. Address queries to Editor of Applied Spectroscopy, 9700 South Cass Avenue, Argonne, Illinois 60439.

Spectroscopist: 20 years experience, basic research in atomic spectroscopy and spectrochemistry. Teaching experience at the undergraduate and graduate level. Speaks Italian, Spanish, and French fluently. Seeks responsible inductrial or academic position. Will relocate. Address queries to Editor of Applied Spectroscopy, 9700 South Cass Avenue, Argonne, Illinois 60439.

Chemist-Chemical Engineer: Many years experience super- vision of labs for fine chemicals development, including process machine development. Also scanning electron microscope and AA analysis. Several years in electronics applications. Several years part-time lecturer in a university. Seeks position as environ- mental sciences engineer (pollution control) or as lecturer in chemistry. Will relocate. J. Wright, 2032 Calle Serena, Fullerton, California 92633.

498 Volume 25, Number 4, 1971